All of them have announced that they're running for president or have formed exploratory committees. As for who they are:
• Gravel (D) is a former senator from Alaska (he's announced)
• Hunter (R) is a representative from California (announced)
• Gilmore (R) is a former governor of Virginia (exploratory committee)
• Tancredo (R) is a former representative from Colorado (exploratory committee)

And you thought it was all about Hillary, Obama, McCain, and Giuliani, didn't you?